Group Creates Localized Non-Diffracting Surface Wave

In an article published this weekend in Physical Review Letters, Capasso group researchers report a localized non-diffracting surface wave solution of the wave equation. Named the cosine-Gauss beam for its field intensity profile, the solution, the two-dimensional equivalent of a Bessel beam, is a superposition of two intersecting plane waves modulated by a Gaussian envelope.

The beam is notable for its high intensity central lobe, which, despite being strongly localized, does not diffract during propagation. The group also notes a high degree of tunability in the width and intensity of the central lobe, making the beam suitable for a wide range of coupling applications.

In the lab the wave was formed by laser illumination of two sets of parallel grooves on the surface of a gold film. The sets of grooves, offset from each other by a small angle, each launch a plane wave surface plasmon. The two plane waves interfere along the axis of symmetry to form the propagating cosine-Gauss beam. The investigators were then able to tune the width and peak intensity of the beam by modifying the angle offset between the two sets of grooves. The beam was observed to retain its shape for its full 80 micron propagation distance, until absorptive losses in the gold became appreciable.

This work was headed by Jiao Lin and Patrice Genevet in the Capasso group, and was done in collaboration with the Laboratoire Interdisciplinaire Carnot de Bourgogne in France.